In order to improve a luminance signal and chrominance signal separation (hereinafter referred to as Y/C separation) property, TV sets and/or video tape recorders a three-dimensional Y/C separation circuit adaptively responsive to the motion of a picture pattern. In these motion adaptive Y/C separation circuits, a detection of the motion of picture pattern and an operation to adapt thereto have been large factors in improving the Y/C separation property. The motion picture pattern is detected according to a frame difference (frame non-correlation) of successive frames of an input composite video signal. While the Y/C separation is carried out using a line correlation of lines in a picture pattern determined as a moving picture pattern (hereinafter, referred to as moving picture) and using the frame correlation between two frames of a picture pattern determined as a still picture.
Referring now to FIG. 1, a conventional video signal processing apparatus will be briefly described. The conventional apparatus is provided with such a motion adaptive Y/C signal separation circuit 15. The circuit 15 in FIG. 1 is shown as an LSI circuit, for example, which is discussed in a report titled as "Development of Three-Dimensional YC Processing LSI" by Miyazaki et al at "1989 National Meeting of the Television Society, pp. 215-216.
The composite video signal (hereinafter referred to as NTSC signal) input through the input terminal 1 is applied to an A/D converter 2, then converted into a digital signal. Thus the digital composite video signal is applied to a moving picture Y/C separation circuit 3 in the Y/C separation LSI 15. The moving picture Y/C separation circuit 3 takes continuous three line signals using two 1 H delay circuits 4 and 5 ("H" means the horizontal period) to perform a Y/C separation for moving pictures. That is, the moving picture Y/C separation circuit 3 first detects a vertical correlation of an image from horizontal low band components of the continuous three lines, and then, based on the vertical correlation, mixes two comb filter outputs to yield vertical high band components (line non-correlative components) for three adjacent lines. The vertical high band components extracted by the moving picture Y/C separation circuit 3 are applied to a subtracter 7 and a mixing circuit (hereinafter referred to as MIX) 8 as the chrominance signals of moving pictures (hereinafter referred to the moving picture C), after removing the color carrier band components by a band-pass filter (hereinafter referred to as BPF) 6. The subtracter 7, into which the output from the 1 H delay circuit 4 is also applied, subtracts the moving picture C from the 1 H delayed NTSC signal, so that a luminanee signal Y1 for moving pictures (hereinafter referred to as moving picture Y1) is separated as a remaining output of the subtractor 7. The moving picture Y1 is then applied to another MIX 9.
The NTSC signal delayed by 1 H period in the 1 H delay circuit 4 is applied to a still picture Y/C separation circuit 12 and also to a one-frame delay circuit 10 for delaying signals therethrough for 525 H (i.e., one-frame period). The one-frame delay circuit 10 also applies the delayed signal to another input of the still picture Y/C separation circuit 12 as well as to another one-frame delay circuit 11. Thus the still picture Y/C separation circuit 12 is simultaneously supplied with two NTSC signals, i.e., the present frame NTSC signal from the 1 H delay circuit 4 and a one-frame delayed NTSC signal from the one-frame delay circuit 10. The still picture Y/C separation circuit 12 separates a luminance signal for still pictures (hereinafter referred to as still picture Y) by adding those two input signals and a chrominance signal component (frame non-correlative component) for still pictures by subtracting those two input signals. The still picture Y separated in the still picture Y/C separation circuit 12 is applied to the MIX 9 and the chrominance signal component (frame non-correlative component) is applied to the MIX 8 as the still picture chrominance signal (hereinafter referred to as still picture C) after processed a band limitation through a BPF 13.
The output from the 1 H delay circuit 4, i.e., 1 H delayed signal and the outputs from the one-frame delay circuits 10 and 11, i.e., a one-frame delayed signal and a two-frame delayed signal, are also supplied to a motion detection circuit 14. The motion detection circuit 14 detects the largest one of the moving signals detected based on a difference of one frame and the moving signal detected based on a difference between two frames t and provides the signal o the MIXs 8 and 9. The MIX 9 is supplied with the moving picture Y and the still picture Y and by mixing both of them at a ratio corresponding to the moving signal, provides the luminance signal Y to the output terminal 16. The MIX 8 is supplied with the moving picture C and the still picture C and then mixes both of them at a ratio corresponding to the moving signal to provide the chrominance signal C to the color processing LSI 17.
As described above, the moving picture Y/C separation circuit 3 separates the moving picture C and the moving picture Y through the Y/C separation using the three lines correlation while the still picture Y/C separation circuit 12 separates the still picture C and the still picture Y through the Y/C separation using the frame correlation.
Adoption of such a Y/C separation (motion adaptive Y/C separation) using the frame correlation enables it to improve the Y/C separation property higher than that using the intra-frame correlation. Further, the inter-frame operation for separating the still picture Y and the still picture C composes the frame non-cyclic noise reduction using the non-correlativity of noise. That is, the still picture Y/C separation circuit 12 has such merits that it carries out the Y/C separation and at the same time, reduces frame non-correlative noise and improves the S/N by 3 dB.
However, if the receiving condition for TV broadcast signals is lowered and the S/N of input composite video signals becomes remarkably decreased, the S/N improving effect of 3 dB is still insufficient. In this case, the noise may be erroneously detected as a signal representing a motion picture and the S/N improving effect by the still picture Y/C separation circuit drops. If it is tried to improve the S/N by performing the noise reduction by the frame operation after the frame Y/C separation, increases in the frame memory capacity and the size of the large scale circuit integration can result.
So, if the S/N of the input composite video signal has been deteriorated, the S/N may be recovered by a cyclic noise reduction circuit for the frame operation in the still picture Y/C separation, formed by a series connection of the moving picture Y/C separation circuit and the still picture Y/C separation circuit. In the frame cyclic noise reduction circuit, the S/N improving effect of 8 dB can be attained by setting constants properly. If the S/N is relatively satisfactory, sufficient Y/C separation property is obtained when the moving picture and still picture Y/C separation circuits are connected in parallel with each other as shown in FIG. 1. Further, if the S/N is relatively worse, a cyclic noise reduction circuit should be composed by performing the frame operation of the still picture Y/C separation circuit by connecting the moving picture and still picture Y/C separation circuits in parallel with each other.
However, a signal delay time for the moving picture Y/C separation circuit and the still picture Y/C separation circuit connected in series is longer than that those connected in parallel with each other. Because of this difference in signal delay times, phases of chrominance signal and luminance signal become discontinuous when the connecting state of the Y/C separation circuits is changed. As a result, such problems as color disordering, uneven coloring, color vanishing, etc. are.